<font face="Verdana, Arial" size="2">What if I use premium but don't really need it?
Most cars give optimum performance on regular or mid-grade gasoline. If you're buying premium and your car's not running any better than it does with a lower-octane gasoline, you're probably wasting money. However, some cars may operate better on premium because of additives. Additives, which are found in all gasolines, keep engines clean and make them run more efficiently. Some brands have more effective additives and some use bigger doses in their premium grades.
Do driving habits affect the amount of octane my car needs?

Yes. Driving habits, a vehicle's mileage, and climate and geography can affect how gasoline performs in your car. Octane requirements tend to increase with mileage, at least through the first 15,000 to 20,000 miles. And a car hauling heavy loads over hills requires more octane than the same car driving on level roads. On the other hand, many older vehicles need less octane at higher altitudes, such as the Rocky Mountains.

How do I know which grade of gasoline to buy?

The octane level recommended by your vehicle's manufacturer is a good starting point. But, price, driving habits and personal preference also are important. And so are individual vehicle characteristics. For example, research shows that cars the same age with identical engines have different octane requirements, probably due to manufacturing tolerances. The best advice is: try different gasolines, observe how they perform in your car, and pick the one that meets your needs.</font>

<font face="Verdana, Arial" size="2">6.3 What fuel property does the Octane Rating measure?
The fuel property the octane ratings measure is the ability of the unburnt
end gases to spontaneously ignite under the specified test conditions.
Within the chemical structure of the fuel is the ability to withstand
pre-flame conditions without decomposing into species that will autoignite
before the flame-front arrives. Different reaction mechanisms, occurring at
various stages of the pre-flame compression stroke, are responsible for the
undesirable, easily-autoignitable, end gases.

During the oxidation of a hydrocarbon fuel, the hydrogen atoms are removed
one at a time from the molecule by reactions with small radical species
(such as OH and HO2), and O and H atoms. The strength of carbon-hydrogen
bonds depends on what the carbon is connected to. Straight chain HCs such as
normal heptane have secondary C-H bonds that are significantly weaker than
the primary C-H bonds present in branched chain HCs like iso-octane [21,22].


The octane rating of hydrocarbons is determined by the structure of the
molecule, with long, straight hydrocarbon chains producing large amounts of
easily-autoignitable pre-flame decomposition species, while branched and
aromatic hydrocarbons are more resistant. This also explains why the octane
ratings of paraffins consistently decrease with carbon number. In real life,
the unburnt "end gases" ahead of the flame front encounter temperatures up
to about 700C due to compression and radiant and conductive heating, and
commence a series of pre-flame reactions. These reactions occur at different
thermal stages, with the initial stage ( below 400C ) commencing with the
addition of molecular oxygen to alkyl radicals, followed by the internal
transfer of hydrogen atoms within the new radical to form an unsaturated,
oxygen-containing species. These new species are susceptible to chain
branching involving the HO2 radical during the intermediate temperature
stage (400-600C), mainly through the production of OH radicals. Above 600C,
the most important reaction that produces chain branching is the reaction of
one hydrogen atom radical with molecular oxygen to form O and OH radicals.

The addition of additives such as alkyl lead and oxygenates can
significantly affect the pre-flame reaction pathways. Antiknock additives
work by interfering at different points in the pre-flame reactions, with
the oxygenates retarding undesirable low temperature reactions, and the
alkyl lead compounds react in the intermediate temperature region to
deactivate the major undesirable chain branching sequence [21,22].

The antiknock ability is related to the "autoignition temperature" of the
hydrocarbons. Antiknock ability is _not_ substantially related to:-
1. The energy content of fuel, this should be obvious, as oxygenates have
lower energy contents, but high octanes.
2. The flame speed of the conventionally ignited mixture, this should be
evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other factors
that are far more important. ( such as compression ratio, stoichiometry,
combustion chamber shape, chemical structure of the fuel, presence of
antiknock additives, number and position of spark plugs, turbulence etc.)
Flame speed does not correlate with octane.</font>

<font face="Verdana, Arial" size="2">higher octane gas needs hotter spark to burn as completely as it would take lower octane gas with stock spark... the spark will still ignite fine, just the gasoline won't burn as efficiently

so unless you are running much higher compression ratio, advance your timing by a lot, turbocharging/supercharging, or whatever means to increase power output that you need to worry about detonation, you do not need to run higher octane gas than what the manufacturer suggests

that said, i don't think running 92-octane would decrease performance enough to be able to be felt by butt-dyno... but like what Saw Jai said, your mpg is likely to go down noticeably compare to running 87-octane.</font>

<font face="Verdana, Arial" size="2">The octane rating of the gas actually indicates the anti-knock characteristics of the gas. The higher the octane rating, the lower the volitility of the gas. Higher octane gas actually burns slower and cooler, lower the occurance of pre ignition. Higher octane is needed in cases of cars with a high compression ratio, or cars tuned with timing more advanced than originally designed. Basically, if your not experiencing pre ignition, using higher octane fuel will give no benefit(it's like wearing bigger sneakers in order to run faster).</font>

<font face="Verdana, Arial" size="2">-higher octane contians more additives/unit volume than lower octain. this means that per unit volume, you have less combustible molecules in higher octane gas. this is why your mileage gets worse, because the car needs to use more volume of fuel for every ignition at higher octane to get the same power than from lower octane

-running higher octane gas doesn't hurt your engine, unless the liners are sensitive to the anti-knock additives. depending on the ecu, you can get more power. by that i mean some intelligent ecus will advance timing to take advantage of the higher octane rating and hence make more power. as far as ignition, the mixture will still ignite whenever the spark comes along, so don't even think its an issue.

-some may argue that the additives in higher octane gas will clog or burn poorly. fact is most additives are quite volatile at the ignition temperature, so they will clear the combustion chamber and not leave deposits or clog anything.

-the egg smell is from fuel being burnt in the cat and not in the combustion chamber. specifically the cat right at the exit of the exhaust header, which is right in front of the firewall, and hence its smell can easily carry into the cockpit. the smell means you are running rich. the only way your car can do that is by having ecu meter too much fuel. which means that it has not redjusted to the new fuel yet. unless you reset the ecu, this may take a longer time.

-in genral lower octane gas contains more sulfur. even sunoco advertises this as a reason to buy thier high octane stuff. increase in sulfur content will increase the smell of the rotten eggs.</font>

Originally posted by BLUE2.2VTEC
God that's an old post!!!!!!